This invention relates to rotary compressors and more particularly to rotary gas compressors which employ the principle of injecting a lubricant into the compressor to both cool the compressor and provide a seal within the compressor. The invention is particularly applicable to gas compressors commonly known as screw compressors.
Prior to the present invention it was known to compress gas by rotating a pair of meshing lobed rotors within a casing and supplying lubricant to that casing for maintaining a seal between the rotors and cooling the machine. An early disclosure showing such an arrangement is that shown in U.S. Pat. No. 3,073,513, issued to W. Bailey.
Also known prior to the present invention was the need to regulate the output of a gas compressor. It is common practice in the gas compressor art to provide a receiver storage tank at the outlet of the compressor. For example, if the compressor is being used to supply 100 psi air the compressor will operate until the pressure in the storage tank reaches or is slightly above the 100 psi level. At that point the compressor will conventionally be "unloaded" by closing or reducing the size of the gas inlet of the compressor so that the amount of air compressed by the machine will be reduced. The compressor will operate in this manner until the storage tank falls below the desired level of 100 psi at which time the compressor inlet will again be opened and the compressor will operate in a "loaded" condition.
With many types of compressors, the inlet of the compressor is controlled by an intake regulator which is an open-closed or on-off regulator. With this type of regulator, when the machine is being operated in a loaded condition, the intake regulator fully opens the compressor inlet and the compressor operates at full capacity. When the compressor is being operated in an unloaded condition the intake regulator closes the compressor inlet and the compressor operates at essentially zero capacity. The use of an on-off type intake regulator has the advantage that when the compressor is being operated in an unloaded condition, no gas to be compressed is supplied to the inlet of the compressor and as a result, no gas is being compressed and the only work being done by the driving motor is to simply turn the machine. As a result there is a substantial power saving during unloaded operation. Thus, with an on-off intake regulator operation of the compressor is either at a fully loaded or fully unloaded condition.
Various attempts have been made to devise modulating regulators wherein as the demands on the compressor decrease, there is a gradual closing of the compressor intake to thereby achieve a gradual reduction in the output of the machine. With oil flooded screw compressors it has been common practice to use a modulating regulator which throttles the compressor intake as shown in U.S. Pat. No. 3,105,630 and others, or to use a slide valve such as shown in U.S. Pat. Nos. 3,088,659 and 3,314,597 as well as many others. Although a modulating regulator which throttles the compressor intake has resulted in an adequate control of capacity it has not resulted in a reduction of the power consumed by the machine. This is because the motor still operates to drive the compressor and work is still being done within the compressor. As a result, in most cases where suction throttling is used the power consumption of a screw compressor is reduced by only 25% to 30% even when the compressor delivery is reduced to zero; i.e., operating at fully unloaded conditions. Where a slide valve is used, the delivery of the compressor cannot normally be reduced to zero and the power consumption is reduced by only about 50%.
One of the problems encountered in using an on-off intake regulator with a screw compressor is with the lubrication system which is necessary with a screw compressor. With oil flooded screw compressors, some lubricant must be continuously supplied to the compressor for lubrication and cooling purposes even while running unloaded. However, if too much lubricant is supplied to the machine, the amount of power consumed is not reduced the desired amount. In addition, many lubrication systems for screw compressors rely on the air pressure in a receiver separator tank or a full capacity pump taking its suction from the receiver separator tank to supply lubricant to the compressor. If this receiver separator tank is not vented, air which is in the compressor must be forced against the full pressure of the receiver separator and the power requirements of the machine in the unloaded condition will not be reduced the desired amount. If a full capacity pump is used to supply lubricant to the compressor, the receiver separator tank could be vented, but this is usually a large tank and venting this tank for purposes of capacity control would be impractical.
Various control systems for screw compressors are known in the prior art wherein on-off intake regulators are employed. On such arrangement is shown in U.S. Pat. No. 3,482,768 to Cirrincione et al, wherein a receiver separator is vented and a reduced amount of lubricant is supplied to the machine during unloaded operation. A valve arrangement is shown for reducing the lubricant supplied to the machine during unloaded conditions. In addition, a purge system is used during unloaded operation to remove lubricant from the compressor. The arrangement shown in this patent was also generally known prior to that invention as illustrated by U.S. Pat. No. 2,234,462 to Boldt, wherein a metering pump is used to supply lubricant to the compressor and the oil reservoir is vented during unloaded operation to reduce the lubricant supplied to the compressor during unloaded operation. Similarly, U.S. Pat. No. 2,458,284 shows a valving mechanism for reducing coolant supplied to a compressor when the machine is operating at unloaded conditions or reduced capacity.
Also in the prior art is U.S. Pat. No. 3,936,249 issued to Sato which shows an oil flood screw compressor employing an on-off type intake regulator. In this apparatus power consumption is allegedly reduced by the compressor operating against a reduced pressure at its discharge. Apparently, however, there is no reduction in the lubricant supplied to the compressor during unloaded operation. Thus, the power consumed during unloaded operation is not reduced as much as would be desired. One disadvantage is that the time required to unload each of the prior art compressors is significant. In actual field conditions, a compressor may load and unload as often as four times a minute. With this frequency of loading and unloading, in order to achieve a real power savings, it is necessary for the actual unloading to take place in a very brief period of time. With the present invention, it is believed that the power requirements of the machine can be reduced to 25% of loaded power almost instantaneously and to 15 - 20% based upon energy consumed by the motor in a matter of 3 to 4 seconds.